Popular construction includes the use of dry wall or gypsum board sections secured to wall studs at corners and joints along the wall. The corners and joints are smoothed by application of cement or mastic such as with those tools disclosed by R. G. Ames in U.S. Pat. Nos. 2,824,442, 2,984,857 and 3,888,611. The disclosed tools are improvements over the previous method which was manual application of the cement by means of trowels or trowel-like devices. Ames' devices, however, are not adaptable to finish an outside corner, that is one which is formed at an angle of approximately 270.degree.. Therefore, said corners which conventionally include a galvanized perforated beaded member well known in the plastering art, are usually finished by means of manual application of the cement to the corner with a trowel. To finish such an outside corner in a satisfactory manner requires the skill of a plasterer and a substantial amount of hand labor, the elimination of which is a prime reason that dry wall is so popular in residential home construction. Therefore, there has been a need to provide a tool which is relatively easy to operate which can finish an outside corner quickly with relatively little skill being required by the operator and at the same time be relatively economical so that its use is cost effective when compared to hand plastering.